Conventional internal combustion engines may be equipped with fuel injectors used to provide fuel to the cylinders of the engine, under control of an Electronic Control Unit (ECU). The operation of an injector is controlled by means of a nominal characteristic curve correlating Energizing Time values with corresponding injected Fuel Quantity values.
In practice, the ECU determines the fuel quantity to be injected by the fuel injector, reads from the nominal characteristic curve a corresponding value of the energizing time and finally operates the fuel injector with that energizing time. This nominal curve is referred to a sample injector, operating in nominal conditions and generally such curve is supplied by the injector's manufacturer.
Especially in case of small quantity fuel injections, such as for example pilot injections, the quantity of fuel actually injected by a fuel injector may deviate from a target or nominal quantity thereof defined by the nominal curve, due to production drift or to ageing drift of the injector.
Learning control methods are known to obviate to such issue, such as Small Quantity Adjustment learning (SQA), which is a method aimed to determine an actual characteristic curve of the fuel injector, which can be used instead of the nominal curve to operate the injector at least while performing small fuel injections. These learning control methods generally provides for operating the injector to perform a number of test fuel injections having different predetermined energizing times, of measuring an actual quantity of fuel injected during each one of the test injections to obtain a corresponding point of the actual characteristic curve, and finally of modelling the actual characteristic curve using a linear regression formula based on the obtained points. However, the above described method may be affected by non-linearity errors if the level of injector's drift or fault is high with respect to the test injection.
Accordingly, it is desirable to provide a strategy that improves the determination of an actual characteristic curve of a fuel injector, avoiding non-linearity errors.